When Adobe decided to add metadata to its Photoshop product in about 1995 they were looking around for appropriate standards - and adopted IPTC's Information Interchange Model (IIM) as the best solution.

IIM was created in 1991 as a rich set of metadata for all kinds of content: text, photo, graphics, audio and even video streams. This may have made the decision easy to implement a subset of IIM into Photoshop. From that time millions of digital images have been annotated by using this technology: Photoshop provided the user interface, how metadata fields are saved and stored in the image file was under the control of IIM.

Soon after that time other imaging software got aware of the value of metadata and added features to read, edit and write metadata fields from and to the image file.

The IIM standard is still supported by the IPTC and has its own section on this website.
It includes the written specifications for IIM and a formal XMP Schema for IIM version 4.1.

Further the IPTC maintains a list of software which claims to be compatible with the IIM implementation of photo metadata.

As the IPTC acknowledges the limitations of IIM it developed with Adobe the IPTC Core schema in 2004 which is equivalent to the IIM fields on a semantic level but is built on latest technology, called XMP. In 2008 the IPTC Core was extended by the IPTC Extension schema.